With recent development of information society, high-speed and large-capacity data communications have been required. As a technique for meeting this requirement, an optical communication method is most expected as the effective technique. Also an optical communication network keeps on spreading.
The optical communication is categorized into: a long-distance communication which allows a city to be connected to other city; a middle-distance communication which allows a point to be connected to other point in a single area; and a short-distance communication which allows a device to be connected to other device or allows a section to be connected to other section in a single device.
An example of the short-distance communication is a communication based on a small device such as a mobile phone or the like. A wiring of a current mobile phone is mainly an electric wiring, and a flexible substrate made of polyimide is mainly used therein. Higher-traffic communication will increase in the future, so that higher-speed and larger-capacity transfer of optical signals will be required. Accordingly, a communication method using a film-type light guide will be mainly adopted.
The light guide is an optical component including: a core portion; and a clad portion which covers the core portion and which is adjusted so as to have a lower refraction factor than that of the core portion. Light is kept inside the core portion so as to be guided to a predetermined position.
In a small device such as a mobile phone or the like, various components are disposed with high density, so that it is often that a light guide has to be wired on surfaces of various components to save the space therein. Thus, the film-type light guide is required to be bendable with a small curvature radius.
As the film-type light guide which is bendable with a small curvature radius, for example, there is disclosed a film-type light guide arranged so that a concave is formed on a surface of a clad layer so as to extend in a direction orthogonal to a longer-side direction of a core portion (Patent Literature 1). Such a film-type light guide is produced as follows. On a sacrifice layer formed on a substrate by a stamper, a core portion resin or a clad portion resin is formed, and the resin is cured by irradiation of an ultraviolet ray, and then the sacrifice layer is dissolved.